feat: populate note files with problem descriptions and code stubs

Add populate-notes.mjs that fetches problem descriptions and
Python/C++ code stubs from LeetCode's GraphQL API. Populated
all 197 NeetCode 150 note files with:
- Problem description (examples, constraints)
- Python code stub (function signature)
- C++ code stub (function signature + includes)

API responses cached in leetcode/.cache/leetcode/ for instant re-runs.

org/study_deck_02/dsa/2-d-dynamic-programming/0494-target-sum.org

@@ -1,18 +1,66 @@
 #+PROPERTY: STUDY_DECK_02
 * TODO 0494. Target Sum :medium:
 :PROPERTIES:
-:NEETCODE: [[file:../../roadmap.org::*0494. Target Sum][Roadmap]]
+:NEETCODE: [[file:../../roadmap.org::*0494. Target Sum][0494. Target Sum]]
 :END:
 
+You are given an integer array ~nums~ and an integer ~target~.
+
+You want to build an *expression* out of nums by adding one of the symbols ~'+'~ and ~'-'~ before each integer in nums and then concatenate all the integers.
+
+ - For example, if ~nums = [2, 1]~, you can add a ~'+'~ before ~2~ and a ~'-'~ before ~1~ and concatenate them to build the expression ~"+2-1"~.
+
+Return the number of different *expressions* that you can build, which evaluates to ~target~.
+
+*Example 1:*
+
+
+#+begin_src
+Input: nums = [1,1,1,1,1], target = 3
+Output: 5
+Explanation: There are 5 ways to assign symbols to make the sum of nums be target 3.
+-1 + 1 + 1 + 1 + 1 = 3
++1 - 1 + 1 + 1 + 1 = 3
++1 + 1 - 1 + 1 + 1 = 3
++1 + 1 + 1 - 1 + 1 = 3
++1 + 1 + 1 + 1 - 1 = 3
+#+end_src
+
+
+*Example 2:*
+
+
+#+begin_src
+Input: nums = [1], target = 1
+Output: 1
+#+end_src
+
+
+*Constraints:*
+
+ - ~1 <= nums.length <= 20~
+
+ - ~0 <= nums[i] <= 1000~
+
+ - ~0 <= sum(nums[i]) <= 1000~
+
+ - ~-1000 <= target <= 1000~
+
 ** TODO Approach
 Write your approach here.
 
 ** TODO Python
 #+begin_src python
-
+class Solution:
+    def findTargetSumWays(self, nums: List[int], target: int) -> int:
 #+end_src
 
 ** TODO C++
 #+begin_src cpp
-
+class Solution {
+public:
+    int findTargetSumWays(vector<int>& nums, int target) {
+        
+    }
+};
 #+end_src